Add support for multiple interfaces

example/src/main.nub

@@ -5,28 +5,34 @@ interface Printable
     func print()
 }
 
-struct Human : Printable
+interface Stringable
+{
+    func str(): cstring
+}
+
+struct Human : Printable, Stringable
 {
     name: cstring
 
+    func str(): cstring
+    {
+        return this^.name
+    }
+
     func print()
     {
-        puts(this^.name)
+        puts(this^.str())
     }
 }
 
 func main(args: []cstring): i64
 {
-    let human = alloc Human {
+    let human: Stringable = alloc Human {
         name = "oliver"
     }
 
-    human.print()
-    print_printable(human)
+    puts(human.str())
+    // human.print()
+
     return 0
 }
-
-func print_printable(printable: Printable)
-{
-    printable.print()
-}